Here's a news story st in the Christian Science Monitor that includes this estimate:
"The auction house handling the sale, Heritage Auctions, has valued the medal and diploma at $500,000, which is "an educated guestimate," said Sandra Palomino, Heritage Auctions' director of historical manuscripts. Estimates by Heritage's in-house coin experts went as high as $5 million, Palomino said. [See Photos of Crick's Medal & Other Auction Items]"